This invention relates generally to cavitation-resistant polymer and coating compositions and, in particular, to polymers and coatings that resist the effects of cavitation, especially when applied to propellers, turbines and other mechanical equipment subject to cavitation or other degradative forces.
Cavitation erosion invariably occurs in almost all hydraulic machinery. Generally, cavitation occurs when partial vacuums are formed in a liquid by a swiftly moving body such as a propeller or by high-frequency sound waves. When the head or pressure, acting on water, for example, is reduced to that of vapor pressure (about 1.25 ft absolute head at usual water temperature), flashing of water into vapor (steam) occurs, and voids or cavities form. Under such conditions, slight changes in static pressure or velocity, cause alternate formation and collapse of these cavities, accompanied by an intense local water hammer (the formation of high local momentary pressure). If these cavities collapse on the surface of runner blades or draft tubes, or other such equipment, the pressure generated tends to enter microscopic cracks, causing cavitation erosion (pitting).
Cavitation occurs on a propeller when it revolves faster than water can be supplied to it. The screw then works in a partial vacuum. This may result in marked increase in rpm, slip, and shaft power with little increase in ship speed or effective power. As cavitation develops, noise, vibration, and erosion of the propeller blades, struts, and rudders are experienced. It may occur either on the face or on the back of the propeller. The cavitation bubbles collapse as they move into higher pressure regions toward the trailing edge, causing erosion.
Cavitation and its destructive effects occurs in pumps and turbines due to local pressure drops which generate cavities filled with vapor. These cavities collapse as soon as the vapor bubbles reach regions of higher pressure on their way through the pump. Cavitation may appear along stationary parts of the pump casing or along moving vanes of the impeller. The reduction of the absolute pressure to that of vapor tension may be general (for the whole system) or merely local. The general reduction may be produced by: (1) an increase in the static lift; (2) a decrease in atmospheric pressure; (3) a decrease in the absolute pressure in the system when pumping from a vessel; and (4) an increase in the temperature of liquid. A local decrease in pressure may be caused by dynamic means: (1) an increase in velocity by speeding up the pump; (2) a result of separation and contraction of flow due to a sudden change in direction of flow. The signs of cavitation are: (1) noise and vibration, (2) drop in head-capacity and efficiency curves, and (3) impeller vane pitting. For pumps of low specific speed, the decrease with cavitation in head-capacity characteristics and efficiency is rapid; for medium specific speeds it is more gradual at first and then rapid; for propeller pumps the decrease exists over the whole range of capacity.
Cavitation even occurs in water passages not occupied by steadily flowing water.
To date various means have been proposed to prevent or reduce cavitation and its destructive effects. One approach involves optimizing design. Another involves using cavitation-resisting materials. The metals, for example, most commonly used, in order of their resistance are cast iron, bronze, carbon steel, and stainless steel.
Metallic overlay welding of cavitation resistant weld metals has been found to be a highly successful method for the repair of cavitation damage on hydraulic turbine runners but, this process is costly and time-consuming. Alternatively, a number of compliant polymeric coatings have been used to repair cavitated areas on hydraulic turbine runners.
Space age plastics and alloys, redesign of machines and machine parts, even modification of waterway flow have been tried without lasting effect.